The IP multimedia subsystem (IMS) that is called the third generation network platform provides all the services on the IP basis, differing from the existing circuit-based network system.
The IMS that is the third generation switching system following the circuit switch and the soft switch combines voice and data service into a single structure and transmits the single structure to a device fixed to an IP network or a mobile device.
Also, the IMS has a meaning of changing the network to the open type by using the Internet protocol.
The IMS is a matter of primary concern in the communication field as it increases the flexibility of the wired/wireless network service to accelerate network convergence, it reduces application development and management costs, and it diversifies new services.
The IP network allocates at IP that is usable anywhere in the world, that is, a public IP address to each terminal, and identifies the address of the transmission source during a communication between the terminals.
However, as the IP network application as well as the IMS have been extended, the increase of usage thereof has made the capacity of the public IP addresses very insufficient.
Most of the communication service providers are using the network address translation technology to attempt to ensure sufficient public IP addresses and security of networks of the communication service providers.
The network address translator (NAT) is an address translation device of a communication network translating the private IP addresses into public IP addresses.
That is, the NAT allocates a private IP address available for a local network to the terminal.
When the terminal uses another network or a public Internet, the private IP address allocated to the terminal is changed with a public IP address, and the public IP address is changed with the private IP address.
Also, the port address translation (PAT), which is one of the network address translation technology, changes a port number of a TCP/UDP in addition to the IP address.
Accordingly, the public IP address is used by a plurality of users to thus ameliorate the problem of insufficient public IP addresses.
However, the NAT analyzes Layer 3 and Layer 4 headers of an OSI model and cannot analyze further higher layers, and the NAT cannot support Internet applications including recognition information (an address of the place of dispatch and a port of dispatch) of a packet generating host in a payload within the Layer 4 header of the packet, which is called the problem of the NAT-traversal.
That is, the problem of the NAT-traversal means that the case of unavailable receipt routing is generated from the outside since the recognition information of the packet generating host is set as a receipt address of the terminal within the local network, that is, a private IP address cannot be changed to the public IP address by the NAT.
The simple traversal under UDP through NAT (STUN) has been proposed as a prior art for solving the problem.
The STUN technology is based on the server-client. A STUN server is located outside the NAT to provide the public IP address and port information allocated by the NAT.
According to the STUN technology, respective terminals for driving a STUN-client acquire a technology IP address translated by the NAT from a STUN server.
However, since it is needed to drive the STUN-client for each terminal, it is difficult to apply the STUN technology to the network and the STUN technology acts on the terminal as a load.
Also, a call transmitting terminal may not know whether a call receiving terminal is provided on the same network before a call connection.
In this case, an unneeded process for accessing the STUN server through the NAT is performed even though the call receiving terminal is provided on the network requiring no passing through of the NAT.
Therefore, an undesirable call process is generated and radio channels are wasted.